The present invention relates to a high-carbon steel pipe and a method of producing the steel pipe. More particularly, the present invention relates to a seam welded steel pipe made of high carbon steel which is suitable for use as, e.g., a steering shaft and a drive shaft of automobiles, and a method of producing the steel pipe.
Recently, there has been a keen demand for a reduction in weight of an automobile body from the viewpoint of preservation of the global environment. The program for reducing the weight of an automobile body has hitherto been progressed by replacing steel bars, conventionally used to manufacture parts, with seam welded steel pipes. The use of seam welded steel pipes for parts which have conventionally been manufactured using steel bars, however, causes the following problem with the parts made of high carbon steel, such as a steering shaft and a drive shaft.
The parts made of high carbon steel have hitherto been manufactured from high carbon steel bars into predetermined shapes by cutting. When seam welded steel pipes are used in place of steel bars, the parts cannot often be machined into the predetermined shapes by cutting alone because the seam welded steel pipe has a thin wall thickness. Also, because of being made of high carbon steel, the seam welded steel pipe is poor in cold workability and has a difficulty in cold working, such as swaging and expansion, to obtain the predetermined shape. In view of those problems, a method of joining seam welded steel pipes having different diameters together by pressure welding is proposed, for example, in manufacture of drive shafts. However, that proposed method requires a high production cost in the process of pressure welding, and has another difficulty in ensuring reliability in the joined portion. For those reasons, an improvement in cold workability of seam welded steel pipes made of high carbon steel has keenly been demanded in the art.
A seam welded steel pipe made of high carbon steel is produced by the steps of shaping a steel strip into the form of a pipe by cold roll-forming and then joining adjacent ends of the pipe to each other by electrical resistance seam welding. During those pipe forming steps, not only work hardness is greatly increased, but also the hardness of a seamed portion is increased by the welding, thus resulting in a steel pipe with very poor cold workability. For that reason, it is usual before cold working to heat the produced steel pipe up to the austenitic range and then hold it to stand for cooling, that is, to perform normalizing at about 850xc2x0 C. for about 10 minutes, so that the steel structure is transformed and recrystallized into a structure of ferrite and pearlite. However, a seam welded steel pipe made of high carbon steel and produced by the above conventional method has cold workability that cannot be regarded as sufficient, because it contains pearlite in too large amount. It is said that the range of C content to provide good cold workability has an upper limit of about 0.3%. In a seam welded steel pipe having the C content at such a level, however, sufficient fatigue strength cannot be obtained even if the steel pipe is subjected to heat treatment of hardening and tempering. The seam welded steel pipe is required to have a relatively high value of the C content for providing high fatigue strength.
As one method of producing a steel pipe having high fatigue strength, Japanese Unexamined Patent Application Publication No. 11-77116, for example, discloses a method of producing a steel pipe having high fatigue strength, in which reducing rolling is performed on a base steel pipe, containing C: more than 0.30% to 0.60%, at 400-750xc2x0 C. with an accumulated reduction in diameter of not less than 20%. The invention disclosed in Japanese Unexamined Patent Application Publication No. 11-77116 is intended to perform warm reducing rolling on a base steel pipe to provide high strength with the tensile strength of not less than 600 MPa, thereby increasing the fatigue strength. According to the invention disclosed in Japanese Unexamined Patent Application Publication No. 11-77116, the fatigue strength is surely increased with an increase in tensile strength, but it is not always guaranteed that a high-carbon steel pipe being soft and having superior cold workability is obtained, because the disclosed invention takes an approach of the reducing rolling at relatively low temperatures for an increase in tensile strength.
Also, as a method of producing a steel pipe having high toughness and high ductility, Japanese Unexamined Patent Application Publication No. 10-306339 discloses a method of producing a steel material (steel pipe) having high toughness and high ductility, in which a base material (steel pipe) containing C: not more than 0.60% is subjected to rolling in the temperature range of ferrite recrystallization with a reduction in area of not less than 20%. The invention disclosed in Japanese Unexamined Patent Application Publication No. 10-306339 is intended to make the steel structure finer to produce a structure of fine ferrite, or a structure of fine ferrite+pearlite, or a structure of fine ferrite+cementite, thereby obtaining the steel material (steel pipe) having high toughness and high ductility. With the invention disclosed in Japanese Unexamined Patent Application Publication No. 10-306339, however, crystal grains are made finer to increase the strength and to obtain high toughness and high ductility. To that end, the disclosed invention takes an approach of the reducing rolling at relatively low temperatures for avoiding the crystal grains from becoming coarser. It is hence not always guaranteed that a high-carbon steel pipe being soft and being superior in cold workability and induction hardenability is obtained.
On the other hand, one conceivable method for improving cold workability of a seam welded steel pipe, which has a high value of the C content and provides high fatigue strength, is to anneal the seam welded steel pipe for spheroidizing cementite. However, spheroidization annealing generally requires heat treatment to be performed at about 700xc2x0 C. for a long time of several hours, and therefore increases the production cost. Another problem is that, with spheroidization of cementite, the induction hardenability is reduced and a desired level of strength is not obtained after the heat treatment.
Furthermore, for accelerating the spheroidization of cementite, it is also conceivable to perform the steps of cold working and then annealing of a seam welded steel pipe after normalizing. With this method, lamellar cementite in pearlite is likewise mechanically finely broken into fragments, but dislocations being effective in accelerating dispersion of carbon and serving as precipitation sites of cementate disappear in the process of temperature rise for the annealing. As a result, neither accelerated spheroidization nor fine dispersion of carbides is obtained, and therefore a noticeable improvement in cold workability and induction hardenability is not achieved.
It is an object of the present invention to solve the above-mentioned problems in the related art, and provide a seam welded steel pipe made of high carbon steel, which has superior cold workability and induction hardenability, and a method of producing the steel pipe.
With the view of solving the above-mentioned problems, the inventors have conducted intensive studies for an improvement in induction hardenability of a high-carbon steel pipe containing spheroidized cementite. As a result, the inventors have found that, by carrying out reducing rolling on a seam welded steel pipe made of high carbon steel at least in the temperature range of (Ac1 , transformation point xe2x88x9250xc2x0 C.) to Ac1 transformation point with an accumulative reduction in diameter (referred to also as an xe2x80x9ceffective reduction in diameterxe2x80x9d in the present invention) of not less than 30%, a structure containing cementite with diameters of not greater than 1 xcexcm finely dispersed in ferrite is created in not only a matrix material but also a seamed portion, whereby the structure is softened and lowering of the induction hardenability can be suppressed. Also, the inventors have found that a high-carbon steel pipe thus produced has such a high r-value in the longitudinal direction as which has not been obtained in the past.
A mechanism, based on which the structure containing cementite with diameters of not greater than 1.0 xcexcm finely dispersed in ferrite is created by carrying out reducing rolling at least in the temperature range of (Ac1 transformation point xe2x88x9250xc2x0 C.) to Ac1 transformation point with a higher reduction is not yet clarified in detail, but the view of the inventors on that point is as follows.
In the case of steel having the structure of ferrite+pearlite, lamellar cementite in the pearlite is mechanically finely broken into fragments due to work applied during the reducing rolling. On that occasion, since the temperature is sufficiently high and dispersion is accelerated due to the work, the fragmented cementite is quickly changed into the spherical form that is stable from the standpoint of energy. Consequently, the cementite can be spheroidized in such a short time as that has been impossible to realize with conventional simple annealing, and fine dispersion of the cementite can be achieved.
On the other hand, where a steel pipe under the reducing rolling has the martensite structure as in a seamed portion, martensite is decomposed into ferrite and spherical carbides due to heating and work. On that occasion, precipitation of the carbides is accelerated due to the work and a larger number of precipitation sites are generated. Consequently, cementite can be spheroidized in a short time, and a structure containing cementite spheroidized and finely dispersed therein can be obtained.
Further, where the heating temperature prior to the reducing rolling is set to a level not lowerer than the Ac1 transformation point so that a steel pipe under the reducing rolling has a structure of ferrite and super-cooled austenite, the super-cooled austenitic structure is decomposed into ferrite and spherical carbides due to the work. On that occasion, precipitation of the carbides is accelerated due to the work and a larger number of precipitation sites are generated. Consequently, a structure containing cementite spheroidized in a short time and finely dispersed therein can be obtained.
The view of the inventors regarding a mechanism, based on which a high r-value is obtained by carrying out reducing rolling in the temperature range of (Ac1 transformation point xe2x88x9250xc2x0 C.) to Ac1 transformation point with a higher reduction, is as follows.
By carrying out the reducing rolling on a base steel pipe in the temperature range of (Ac1 transformation point xe2x88x9250xc2x0 C.) to Ac1 transformation point, in which the structure is primarily ferrite, with an accumulated reduction in diameter of not less than 30%, an ideal aggregation structure due to the rolling, in which the  less than 110 greater than  axis is parallel to the longitudinal direction of the pipe and the  less than 111 greater than  to  less than 110 greater than  axes are parallel to the radial direction thereof, is formed and then further developed through restoration and recrystallization. The aggregation structure due to the rolling produces very great driving forces because crystals are rotated by working strains. Unlike an aggregation structure that is created through recrystallization in the case of obtaining a high r-value in steel sheets, the aggregation structure due to the rolling is less affected by the second phase and the amount of solid solution carbon. Consequently, a high r-value is obtained even for a seam welded steel pipe made of high carbon steel, although such a high r-value has been difficult to realize in steel plates made of high carbon steel. Note that the above-mentioned effect is specific to the reducing rolling. In other words, the effect of providing a high r-value is developed because the drafting force is applied in the circumferential direction in the reducing rolling. Conversely, the r-value is reduced in plate rolling, for example, because the drafting force is applied in the thickness direction of a plate.
The present invention has been accomplished based on the findings described above.
According to a first aspect of the present invention, there is provided a high-carbon steel pipe having superior cold workability and induction hardenability, wherein the steel pipe has a composition containing, by mass %, C: 0.3 to 0.8%, Si: not more than 2%, and Mn: not more than 3%, or, as required, Al: not more than 0.10%, the balance consisting of Fe and inevitable impurities, and the steel pipe has a structure with the grain size of cementite being not greater than 1.0 xcexcm at any positions including a seam. In the high-carbon steel pipe according to the first aspect, preferably, the steel pipe further contains in addition to the aforesaid composition, by mass %, one or more selected from among Cr: not more than 2%, Mo: not more than 2%, W: not more than 2%, Ni: not more than 2%, Cu: not more than 2%, and B: not more than 0.01%. Also, in the high-carbon steel pipe according to the first aspect, preferably, the steel pipe further contains in addition to the aforesaid composition, by mass %, one or more selected from among Ti: not more than 1%, Nb: not more than 1%, and V: not more than 1%.
Further, in the high-carbon steel pipe according to the first aspect, preferably, an r-value is not less than 1.2 in the longitudinal direction of the steel pipe at any positions including the seam.
According to a second aspect of the present invention, there is provided a method of producing a high-carbon steel pipe having superior cold workability and induction hardenability, the method comprising the steps of preparing a base steel pipe having a composition containing, by mass %, C: 0.3 to 0.8%, Si: not more than 2%, and Mn: not more than 3%, or, as required, Al: not more than 0.10%, the balance consisting of Fe and inevitable impurities; and carrying out reducing rolling on the base steel pipe at least in the temperature range of (Ac1 transformation point xe2x88x9250xc2x0 C.) to Ac1 transformation point with an accumulated reduction in diameter of not less than 30%.
Also, in the method of producing the high-carbon steel pipe according to the second aspect, preferably, the steel pipe further contains in addition to the aforesaid composition, by mass %, one or more selected from among Cr: not more than 2%, Mo: not more than 2%, W: not more than 2%, Ni: not more than 2%, Cu: not more than 2%, and B: not more than 0.01%. Also, in the method of producing the high-carbon steel pipe according to the second aspect, preferably the steel pipe further contains in addition to the aforesaid composition, by mass %, one or more selected from among Ti: not more than 1%, Nb: not more than 1%, and V: not more than 1%.
Further, in the method of producing the high-carbon steel pipe according to the second aspect, preferably, the base steel pipe is a seam welded steel pipe produced by the steps of slitting a steel strip into a predetermined width, removing droops in slit surfaces, and joining the slit surfaces to each other by electrical resistance seam welding.